How to edit video with a third audio channel
You added information to your original question. It's not so important how many audio channels the video has in your scenario, you want to drop the volume of one of the audio tracks for a cough/sneeze/burp you can keyframe the volume of a track.




view the audio track as an audio waveform
set keyframes at the appropriate places. A sane volume range is between 0.0 and 1.0